1. Field of the Invention
The present invention relates to a method of manufacturing a flexible circuit board comprising a conductor circuit layer and a polyimide insulating layer having land access holes provided on one or both sides of the conductor circuit layer.
2. Related Art of the Invention
Flexible circuit boards can be broadly classified as single-sided flexible circuit boards and double-sided flexible circuit boards, the latter affording higher mounting densities.
Typically, the design of a single-sided flexible circuit board comprises an insulating layer having land access holes, and a conductor circuit layer supported thereon. Optionally, a protective layer is laminated over the conductor circuits. Circuit boards of this design are termed xe2x80x9csingle-access circuit boardsxe2x80x9d since connections are secured from the one side.
Flexible circuit boards of this type are typically fabricated in the following manner. A single-sided copper clad film produced by laminating copper foil to a polyimide support film is subjected to patterning of the copper foil thereof to produce a conductor circuit layer, and a cover layer is then laminated over the conductor circuit layer. Lamination of the cover layer is accomplished by 1) laminating a cover layer film provided in advance with land access holes by means of a punching cutter, etc., and having an adhesive applied thereto; 2) printing a land access hole pattern with an epoxy resist ink; or 3) after laminating the polyimide resin film, forming land access holes in the polyimide resin film by means of an etchant or irradiation with a laser beam.
Double-sided flexible circuit boards, on the other hand, have a design comprising an upper conductor circuit layer/insulating layer/lower conductor circuit layer laminate sandwiched between a pair of cover layers provided with land access holes. Circuit boards of this design are termed xe2x80x9cdouble-access circuit boardsxe2x80x9d since connections are secured from both sides.
Double-sided flexible circuit boards of this type are typically fabricated in the following manner. A double-sided copper clad film having copper foil laminated to both sides of a polyimide film is drilled through with an NC drill to produce holes; the surfaces are covered with a resist film, except for the land areas; an electroless copper deposition film is formed on the hole walls; and the copper film is then thickened through electrolytic copper plating to complete the through-holes. Next, after removing the resist film, the copper foil on each side is patterned by the subtractive process (while protecting the walls of the through-holes) to produce the upper conductor circuit layer and lower conductor circuit layer. Cover layers are then laminated to each of these surfaces in the same manner as with a single-access type single-sided flexible circuit board. This completes the fabrication process for a double-access type double-sided flexible circuit board.
Recently, attempts have been made to use a single-sided flexible circuit board as a double-access circuit board; this is fabricated in the following manner. A polyamic acid varnish is applied to copper foil and dried to give a polyamic acid layer. The polyamic acid layer is patterned by a photolithographic process to produce the land access holes, and the layer is then imidated to produce a polyimide insulating layer. The copper foil is patterned by the subtractive process to produce the conductor circuit layer, and a cover layer is laminated over the conductor circuit layer in the same manner as with a single-access type single-sided flexible circuit board. This completes the fabrication process for a double-access type single-sided flexible circuit board.
Drawbacks of conventional fabrication processes for single-access type single-sided flexible circuit boards include the following: where the cover layer film is provided with preformed land access holes using a punching cutter, patterning of fine lines is difficult, precision of alignment is not satisfactory, and hole shape is limited to a substantially round configuration, limiting the degree of freedom as to shape, and as a result making it difficult to expand the soldering area (land area). Another problem is that when the cover layer film is laminated over the conductor circuit layer, the adhesive is pressed out onto the lands, resulting in less reliable conductivity of the lands. Where lands are particularly small, some lands may become entirely covered by adhesive, with the result that conductivity cannot be assured.
Where lamination of the cover layer is effected by screen printing of a land access hole pattern using an epoxy-based resist ink, it is not possible to produce a film having adequate flexibility due to the presence of an epoxy resin as the principal component. While it would be possible to use a polyimide-based resist ink, high hygroscopicity contributes to poor printability and inadequate controllability of printing thickness. Another problem is that it will be extremely difficult to make finer hole pattern size by using conventional printing techniques.
Where lamination of the cover layer is effected by lamination of a polyimide resin film followed by formation of land access holes in the polyimide resin film by means of wet etching, it will be necessary to use as etchants strongly basic reagents (aqueous alkaline solutions of hydrazines), which are expensive, and the additional costs entailed in etchant disposal will make it difficult to reduce etching costs. Where the land access holes are formed by means of irradiation with a laser beam, productivity tends to be low since holes must be made one by one; further, burned material tends to become deposited on the lands, making necessary treatment with potassium permanganate solution, which contributes to higher production costs.
Problems pertaining to fabrication of double-access type double-sided flexible circuit boards, on the other hand, are, where holes are produced in double-sided copper clad film using an NC drill, the significant equipment costs associated with the NC drilling unit and low productivity due to the fact that holes are made one by one. Other problems are burring at the end surfaces around hole openings and non-uniform hole shape. Another problem is that cuttings adhere to hole walls resulting in less reliable through-hole conductivity. Yet another problem associated with double-sided flexible circuit boards is excessive thickness overall.
Accordingly, attempts have been made to substitute the single-sided flexible circuit boards described previously for double-sided flexible circuit boards for use as double-access type flexible circuit boards; however, since patterning of the copper foil is carried out after imidation of the polyamic acid layer, the problem arises that removal of the copper foil pattern tends to be accompanied by curling of the conductor circuit layer/polyimide film laminate. Also, problems similar to those experienced with the single-access type single-sided flexible circuit boards described earlier occur during cover layer lamination.
The present invention is intended to solve the problems associated with the prior art, and has as an object manufacture of single-access type or double-access type single-sided flexible circuit boards at low cost with fine line patterning, precise alignment, and reduced curling; as well as to increase the degree of freedom as regards hole shape, making it possible to expand the soldering area (land area), and to enable cover layer films or corresponding insulating layers to be produced over conductor circuits layer without adversely affecting land conductivity.
The inventors perfected the invention, which achieves the objects stated above, upon discovering that i) polyimide precursors such as polyamic acids, used in conjunction with photolithographic processes employing inexpensive alkali etchants, afford low cost, fine line patterning, precise alignment, and a high degree of freedom in patterning; ii) polyimide precursors are capable of withstanding conditions used in copper foil patterning; and iii) accordingly, where a polyimide film produced by imidation of a polyimide precursor is employed as an insulating layer in a single-access type or double-access type single-sided flexible circuit board, the copper foil can be patterned prior to imidation of the polyimide precursor layer, allowing a polyimide insulating layer corresponding to a cover layer film to be produced over a conductor circuit layer without adversely affecting land conductivity.
Specifically, the invention provides a method of manufacturing a flexible circuit board comprising a polyimide insulating layer having land access holes and a conductor circuit layer provided thereon, comprising the steps of:
(a) applying a polyimide precursor varnish to one side of a conductor circuit metal foil and drying to produce a polyimide precursor layer;
(b) forming the land access holes in the polyimide precursor layer by a photolithographic process;
(c) patterning the conductor circuit metal foil by the subtractive process to produce the conductor circuit layer; and
(d) imidating the polyimide precursor layer to produce the polyimide insulating layer.
The present invention further provides a method of manufacturing a flexible circuit board comprising a first polyimide insulating layer and a second polyimide insulating layer, each having the land access holes, and a conductor circuit layer sandwiched therebetween, comprising the steps of:
(aa) applying a polyimide precursor varnish to one side of a conductor circuit metal foil and dried to produce a first polyimide precursor layer;
(bb) forming the land access holes in the polyimide precursor layer by a photolithographic process;
(cc) patterning the conductor circuit metal foil by the subtractive process to produce the conductor circuit layer;
(dd) applying a polyimide precursor varnish to the conductor circuit layer and drying to produce a second polyimide precursor layer;
(ee) forming land access holes in the second polyimide precursor layer by a photolithographic process; and
(ff) imidating the first polyimide precursor layer and the second polyimide precursor layer to produce the first polyimide insulating layer and the second polyimide insulating layer, respectively.
These and other objects, features and advantages of the present invention are described in or will become apparent from the following detailed description of the invention.